The science behind the idea has been proven, says Wade Allison, emeritus professor of physics at Keble College, Oxford. The challenge is more practical.

“The timescale we can’t be sure about, but the basic science is solved and the problems are technical ones to do with materials,” says Prof Allison.

Why is it so difficult?

A major challenge is how to build a structure strong enough to contain the plasma – the very high-temperature nuclear soup in which the fusion reactions take place – under the huge pressures required.

Exhaust systems will “have to withstand levels of heat and power akin to those experienced by a spaceship re-entering orbit,” says Prof Ian Chapman, chief executive of the UK Atomic Energy Authority (UKAEA),

Robotic maintenance systems will also be needed, as well as systems for breeding, recovering and storing the fuel.

“High temperature” in the context of this branch of physics means a distinctly chilly -70C or below.

“They’ve been by far the most successful to date,” says Jonathan Carling, the firm’s chief executive.

“A spherical tokamak is a much more efficient topography, and we can drastically improve the compactness and the efficiency. And because it’s smaller, it can be more flexible, and the cost to build is also lower,” he says.

The company has built three tokamaks so far, with the third, ST40, built from 30mm (1.2in) stainless steel and using HTS magnets. This June it achieved plasma temperatures of more than 15 million C – hotter than the core of the sun.

The firm hopes to be hitting 100 million C by next summer – a feat Chinese scientists claim to have achieved this month.

“We expect to have energy gain capability by 2022 and be supplying energy to the grid by 2030,” says Mr Carling.

Meanwhile in the US, MIT [Massachusetts Institute of Technology] is working with the newly-formed Commonwealth Fusion Systems (CFS) to develop Sparc, a doughnut-shaped tokamak with magnetic fields holding the hot plasma in place.

Funded in part by Breakthrough Energy Ventures, a fund led by Bill Gates, Jeff Bezos, Michael Bloomberg and other billionaires, the team hopes to develop fusion reactors small enough to be built in factories and shipped for assembly on site.

Image copyrightGetty ImagesImage caption The Iter nuclear fusion reactor will not be completed until 2025

Iter, which also means “the way” in Latin, is building the biggest experimental fusion facility in the world, but it doesn’t expect to fire up until 2025, and any commercial application will come a long way after that.

“Different Iter members have different levels of urgency for using fusion as part of a clean energy future,” a spokesman tells the BBC.

“Some clearly expect to have fusion electricity to the grid before 2050; for others the roadmap is in the second half of this century.”

The new kids on the block think they can do better.

“With the new HTS magnet technology, a net-energy fusion device can be much, much smaller – Sparc would be about one sixty-fourth the volume and mass of Iter,” says Martin Greenwald, deputy director of MIT’s plasma science and fusion centre.